A photodetecting apparatus is provided with a photodiode for generating an amount of electrical charge corresponding to an amount of incident light and an integrating circuit for accumulating the electrical charge generated by the photodiode to output a voltage value corresponding to an amount of the thus accumulated electrical charge. Further, the photodetecting apparatus may be provided from time to time with an A/D converting circuit for converting a voltage value (analog value) output from the integrating circuit to a digital value, thereby outputting the digital value. Still further, where a plurality of photodiodes are arrayed one-dimensionally or two-dimensionally, the photodetecting apparatus is able to pick up a one-dimensional image or a two-dimensional image.
There has been known an A/D converting circuit having the constitution disclosed in Non-Patent Document 1 as an A/D converting circuit used in the above-described photodetecting apparatus. This A/D converting circuit, which includes at least 2 N-number of capacitors, is able to input an analog value to be subjected to A/D conversion as a differential signal and output an N-bit digital value corresponding to the analog value. In this instance, N is an integer number equal to or greater than two.
In a photodetecting apparatus including the above-described A/D converting circuit, when an amount of electrical charge corresponding to an amount of incident light to photodiodes is accumulated, a voltage value (signal component+noise component) output from an integrating circuit is input into the A/D converting circuit, and when the electrical charge is not accumulated, a voltage value (noise component) output from the integrating circuit is input into the A/D converting circuit. Thereby, a difference between these two voltage values (that is, only the signal component) is converted by the A/D converting circuit to a digital value. Therefore, the digital value output from the A/D converting circuit is a value corresponding to an amount of incident light to photodiodes, that is, a highly accurate value from which the noise component is eliminated.
Non-Patent Document 1: Technical Data Sheet of the product AD7641 available from Analog Devices Inc.